At a Glance

Why Get Tested?

To detect and diagnose Legionella bacteria as the cause of pneumonia or flu-like symptoms; to investigate an outbreak caused by Legionella

When to Get Tested?

When you have a cough, body aches, shortness of breath, headache, and fever with chills, and may have abdominal pain, diarrhea, nausea, and/or mental changes that a doctor suspects may be due to an infection caused by Legionella

Sample Required?

A urine sample, a sputum sample, and/or respiratory secretions or other body fluids collected by a doctor

Test Preparation Needed?

None

The Test Sample

What is being tested?

Legionella is a type of bacteria that can cause a serious form of pneumonia called Legionnaires disease. Legionella testing detects the presence of the bacteria to help diagnose the cause of a person's pneumonia.

A legionella infection affects the lower respiratory tract, often requires hospitalization, and has a significant mortality rate. In addition to pneumonia, Legionella bacteria can also cause Pontiac fever, a milder and self-limiting illness with flu-like symptoms. Together Legionnaires disease and Pontiac fever are referred to as legionellosis.

Legionella bacteria are found naturally throughout the environment. They prefer warm, stagnant water and can grow in the plumbing systems of large buildings such as hotels, hospitals, and cruise ships. The bacteria may contaminate whirlpool spas, drinking and bathing water, hot water tanks, air conditioning cooling towers, ice machines, humidifiers, and fountains.

Infections occur when an individual inhales steam, mist, or airborne droplets contaminated with Legionella bacteria. The infection is not spread from one person to another. Pontiac fever typically develops within 24-48 hours while Legionnaires disease develops within a few days to a couple of weeks. According to the Centers for Disease Control and Prevention (CDC), about 8,000 to 18,000 people are hospitalized with Legionnaires disease each year. The actual number of people affected by legionellosis is likely much higher as most cases of Pontiac fever and many cases of Legionnaires are thought to go undiagnosed.

Although anyone may be affected, adults over 50 are at the highest risk for Legionnaires disease, as are those who smoke, have lung diseases, or have compromised immune systems (such as those with HIV/AIDS, cancer, or who are taking immunosuppressant medications). People with chronic underlying diseases such as diabetes, kidney disease, or liver disease are also more vulnerable, and men are affected more often than women. Most cases of Legionnaires are sporadic, but they may also be seen in outbreaks, and Pontiac fever is frequently linked with outbreaks.

There are many different types of Legionella bacteria, but only a few are common. In the United States, as many as 80% of legionella infections in adults are caused by Legionella pneumophila, serogroup 1 (a subtype of the species). Other serogroups of L. pneumophila, including 4 and 6, cause a number of cases. Other species of Legionella, such as Legionella micdadei, Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae, may cause infections in children and/or are more prevalent in other parts of the world.

A few different types of tests are available to help diagnose infections caused by Legionella bacteria:

An antigen test that detects a protein made by the bacteria in urine is one of the most common tests.

The bacteria may also be detected by culturing sputum, respiratory secretions or other body fluids.

How is the sample collected for testing?

The sample collected depends on the test to be performed.

For culture or molecular testing, a sputum sample may be expectorated or induced. Expectorated samples are coughed up and expelled into a sterile cup provided by the laboratory or doctor's office. The person's mouth should be rinsed with water or saline prior to sample collection. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Respiratory secretions, other body fluid samples, and any tissue biopsy samples are collected by a doctor.

For antigen testing, a random urine sample is collected in a sterile cup provided by the laboratory or doctor's office.

Is any test preparation needed to ensure the quality of the sample?

The Test

How is it used?

Testing is performed to detect the presence of Legionellabacteria so as to identify the cause of a person's symptoms. Tests are primarily used to diagnose Legionnaires disease and to distinguish it from other causes of pneumonia and/or other conditions that have similar symptoms. Since Legionnaires disease can often require hospitalization and has a significant mortality rate, it is important for a doctor to identify it and initiate appropriate treatment with antibiotics.

Typically one or more of the following tests are ordered:

Legionella species by polymerase chain reaction (PCR)—this test detects bacterial genetic material, primarily in respiratory secretions but sometimes in other body fluids. It can detect several species of Legionella.

Legionella pneumophilaantigen—this test detects one of the bacterium's proteins. It is performed on urine and occasionally on another body fluid. It is a rapid way to detect an infection, but it will only detect Legionella pneumophila serogroup 1. Since this is the most common cause of Legionnaires disease in the U.S., it will detect most infections in adults.

Legionella culture—this type of culture may be performed along with a routine sputum culture because special nutrient media is required to encourage the growth of Legionella and discourage the growth of other bacteria. This test is considered the "gold standard" for diagnosing an infection caused by Legionella bacteria. A positive culture may be determined in about 48 to 72 hours. Negative cultures are held for at least 7 days before a final result is reported. A culture will identify multiple Legionella species and is used both to confirm that a person has a legionella infection and to help identify the source of an outbreak.

Other tests may be performed to help evaluate a person's overall health status and to distinguish between conditions that cause similar symptoms. These may include:

When is it ordered?

Legionella testing is ordered when a person has pneumonia or symptoms associated with pneumonia, especially when the person also has gastrointestinal symptoms such as diarrhea and/or nausea and mental changes such as confusion.

What does the test result mean?

Results of testing must be carefully interpreted and evaluated in conjunction with a complete medical history, physical exam, and other tests such as a chest X-ray for pneumonia.

Negative legionella test results, of any type, do not rule out a legionella infection. A person may be infected by a Legionella species other than the one(s) tested or the bacteria may be present in the sample in numbers too low to be detected or may not have grown in culture. Repeat testing may be required.

If a Legionella pneumophila urine antigen test is positive, then it is likely that a person has a Legionella pneumophila serogroup 1 infection. If the person has symptoms consistent with pneumonia, then it is likely that the person has Legionnaires disease.

If a legionella culture is positive, then a person is confirmed as having a legionella infection and whatever species is present in the culture is identified as the cause.

If a Legionella species by PCR genetic test is positive, then it is likely that the person has a legionella infection. However, there are a small number of false positives associated with this test.

Is there anything else I should know?

Legionnaires disease can affect other blood test results. These tests are not used to help diagnose a legionella infection but may be additional clues for the doctor and information about the person's health status. Often seen are:

On a Gram stain slide under the microscope, the presence of white blood cells but few or no bacteria. If Legionella bacteria are present, they typically appears as small, pale, Gram-negative (pink stained) variable-shaped rods.

Tests for Legionella pneumophilaantibody are no longer routinely performed but may be available from some reference laboratories. The test detects antibodies, the body's immune response to a legionella infection, in the blood. It is not useful for immediate clinical decisionmaking but can be ordered to confirm a recent infection. Usually two blood samples taken weeks apart are tested. A single positive antibody test cannot be used to diagnose a recent infection as up to 15% of the general population may have legionella antibodies in their blood, indicating exposure to the bacteria at some point in their life. Some people, such as those who have compromised immune systems, can have a legionella infection without having a normal antibody response.

Legionella pneumophila by direct fluorescent antibody (DFA) is a test that is no longer routinely recommended. It is not as sensitive as other testing, is labor intensive, and requires a relatively large respiratory sample.

1. Can water be tested for Legionella bacteria?

Yes, and it frequently is tested to monitor complex water systems, to detect the source of legionellosis outbreaks, and to monitor the effectiveness of water treatment when Legionella bacteria are detected.

2. Can Legionella bacterial contamination and infection be prevented?

The opportunities for them to occur can be minimized, but the risk cannot be totally eliminated. Legionella are very common bacteria in the environment. In building water systems, they can resist low levels of chlorine treatment and can persist in sediment found in pipes.

4. Why is it called Legionnaires disease?

The disease and bacteria were named after an incident in which a significant number of people who went to an American Legion convention in Philadelphia in 1976 developed pneumonia. A new type of bacteria was identified as the cause and was subsequently named Legionella. This outbreak caused illness in 182 people and resulted in 29 deaths.

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Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

(Modified 2011 June 1). Patient Facts: Learn More about Legionnaires' disease. CDC Legionellosis Resource Site [On-line information]. Available online at http://www.cdc.gov/legionella/patient_facts.htm through http://www.cdc.gov. Accessed April 2012.

(Updated 2011 June 1). Top 10 Things Every Clinician Needs to Know About Legionellosis. CDC Legionellosis Resource Site [On-line information]. Available online at http://www.cdc.gov/legionella/top10.htm through http://www.cdc.gov. Accessed April 2012.

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This article was last reviewed on June 23, 2012. | This article was last modified on October 8, 2014.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.